The present invention relates to apparatus for giving a satin finish to the roll surface of a work in roll form by electric discharge machining, and more particularly the invention relates to an electric discharge machining apparatus for giving a satin finish to the outer surface of a crowned roll with a predetermined surface roughness.
In the past, rolling mill rolls of many different shapes have been put in practical use to suit various thicknesses, widths and mechanical properties of rolled products, drafts, shapes of raw sheet materials, etc. they include a flat roll shown in FIG. 1, a sine-crowned roll in FIG. 2, a step-crowned roll in FIG. 3 and a narrow body crowned roll in FIG. 4, and these rolls are used alternately or each of the rolls is used several times continuously depending on the production schedule of a rolling mill. As a result, with a dull finishing machine which gives a satin finish to the surface of a roll, there occurs with high frequency the situation is high in which a plurality of rolls of different shapes are alternately machined and an electrode changing operation or electrode adjustment is required to suit the shape of a roll to be worked, thus deteriorating the working efficiency and thereby causing loss of time and requiring a large expense for labor.
In the past, various methods have been used to give a satin finish to the surface of crowned rolls by an electric discharge machine for rolls, and in accordance with the first method, as shown in FIG. 5, a preliminarily prepared electrode assembly 12 having the same shape as that of a roll 10 is used and the electrode assembly 12 is shaped by machining or the desired electrode shape is produced by using a dummy roll 10' as shown in FIGS. 6A and 6B. In any case, this method has the disadvantage of poor yield of electrodes since an electrode assembly of the same shape must be prepared to suit each one of different roll shapes and the electrode assembly must be changed to suit a roll of a different shape. Where a dummy roll is used to produce the desired electrode shape, there is a disadvantage in that not only a dummy roll is necessary for every roll but also the resulting crown tends to become somewhat smaller than the desired one since, as shown in FIG. 6B, the amount of change 14 in the roll shape of the dummy roll 10' cannot even theoretically become the same with the amount of change 16 in the electrode shape of the electrode 12 due to the difference in loss between roll 10' and the electrode 12. Another disadvantage of this method is that, since the electrode must be extended along the entire roll body length, the electrode must be in the form of multi-divided electrodes from the standpoint of ensuring a greater working efficiency, and as is well known in the art an insulator 18 must be provided between the electrodes as shown in the sectional view of FIG. 7 taken along the line VII--VII of FIG. 5, thus increasing the cost of working an electrode, decreasing the yield and requiring a complicated setting of the electrode assembly comprising an electrode holder 20, electrode mounting bolts 22 and a servomechanism 24. In addition, as is well known, discharge phenomenon can take place most efficiently at the electrode edge and, consequently, if a roll is machined without moving the electrodes having a sectional electrode shape as shown in FIG. 7, it is impossible to accomplish a uniform electric discharge machining of the entire roll surface and a streaky pattern in the roll surface is caused thus making the roll as such unfit for use as a rolling mill roll. To overcome this difficulty, during the machining of a roll the electrodes are each moved along the axis of the roll within the width of one electrode, this method is also disadvantageous in that moving the electrodes along the axis of the roll makes it impossible to maintain constant the gap between the electrodes and the roll along the entire width of the electrodes, and consequently a complicated mechanism is needed to move the electrodes from side to side along the roll shape instead of moving them along the roll axis. However, even if the electrodes are moved along the roll shape, due to the fact that the electrodes are arranged to oppose the entire range of the roll body length, on one side the electrode will be deviated from the roll surface, and consequently the deviated electrode will not be consumed, thus changing the electrode shape and eventually changing the shape of the roll at its ends as compared with the initial shape. It will thus be seen that the first method has many disadvantages, is difficult to be used in practical applications, and is not desirable from the standpoint of economy and quality of product.
In accordance with the second method, as shown in FIG. 8, a single electrode 12 which is not of a split structure is fed in the lengthwise direction of a roll 10 along the roll shape. The second method has the same disadvantages as the first method in that the method requires a longer working time, thus making it impossible to embody the method in an electric discharge machine capable of satinizing rolling mill rolls with a high productivity, and in that it is necessary to use electrodes which suit the shape of a roll. The second method is also of the type in which a different electrode material is used to make the electrode consumable, and this method is also disadvantageous in that the machining speed of a brass electrode is as low as about half of a copper electrode and the amount of electrode consumption is increased, thus increasing the frequency of electrode change. There are other disadvantages of the brass tending to deposit on the roll surface, a decrease in the number of indentations in the satin finished roll surface, and so on, and use of the second method in practical applications also involves a number of difficult problems.